Recently, piezoelectric laterally vibrating resonators (LVRs) have received a great deal of attention thanks to their small size, high quality factor (Q), low impedance, low cost integration with complementary metal-oxide semiconductor (CMOS) circuitry, and multiple operating frequencies on a single chip. Among the various piezoelectric material platforms, LVRs based on transferred Lithium Niobate (LiNbO3) thin films are promising in meeting demanding specifications of next-generation radio frequency (RF) front-end duplexers, due to their capability to feature high electromechanical coupling (kt2) and high Q simultaneously for various acoustic modes over a wide frequency range. Among these acoustic modes, the shear horizontal (SH0) as well as the symmetrical (S0) modes have both been theoretically predicted and experimentally demonstrated in X-cut, LiNbO3 LVRs with very large kt2, high Q, and a record-breaking figure of merit (FoM).
In spite of their strong potential, LiNbO3 LVRs are still confronted with the bottleneck of having pronounced spurious modes. These spurious modes cause in-band ripples and out-of-band spurious response for the filters, obstructing the LiNbO3 LVRs' deployment as a commercial solution for future radios. The spurious modes in LVRs typically originate from several sources, such as higher order symmetric overtones, asymmetric wave propagation, acoustic wave interaction with bus lines and anchors, and transversely guided standing waves. To overcome this bottleneck, numerous studies have focused on subduing the various spurious modes in the LVRs, especially for the Aluminum Nitride (AlN) devices. A weighted electrode design has also been reported to subdue the S0 mode overtones in LiNbO3 LVRs. However, there has not yet been a report addressing the suppression of transverse spurious modes in LiNbO3 LVRs. In comparison to other LiNbO3 LVRs with various high kt2 modes, the SH0 devices have been demonstrated to feature fewer spurious modes. Nonetheless, SH0 devices are still prone to transverse spurious modes owing to the high intrinsic kt2 of LiNbO3.